Shock-absorbing devices are used in a wide variety of vehicle suspension systems for controlling motion of the vehicle and its tires with respect to the ground and for reducing transmission of transient forces from the ground to the vehicle. A shock absorber accomplishes these functions by both dissipating and storing energy. Energy is stored within the shock absorber by compressing a liquid or a gas within a chamber, which acts as a spring, so that upon termination of a force compressing the shock absorber, the shock absorber is restored to its original uncompressed length. Energy is dissipated by passing a viscous liquid such as oil through an orifice, so that as the shock absorber is compressed or extended, its rate of motion is limited by the damping action of the orifice.
Shock struts which are used in the landing gear of aircraft are subject to more-demanding performance requirements than ground vehicle shock absorbers. Specifically, aircraft shock struts must be able to control motion of the landing gear and absorb and damp loads imposed on the gear during diverse use regimes including landing, taxi, and takeoff. Shock strut design must often be compromised between these diverse regimes, because strut damping and spring rates that are optimum for damping relatively small-amplitude motions experienced during taxi may not be optimum for controlling motion of the landing gear during landing, where significantly greater shock strut piston stroke and dynamic forces occur.
Consequently, shock strut damping may be less than ideal during landing, resulting in aircraft rebound caused by too-rapid spring extension of the shock strut after the initial touchdown and compression of the strut. Rebound is undesirable because aircraft braking efficiency is significantly reduced, leading to greater field length required to bring the aircraft to a stop. Moreover, subsequent ground loads after a rebound may be unnecessarily severe if the aircraft wing lift spoilers are deployed after initial touchdown.
In addition to the problem of rebound, other problems are associated with conventional shock struts for aircraft. For instance, during taxi, it is desirable for the shock strut to allow piston excursions caused by the tires encountering bumps or depressions in the runway, without transmitting significant forces to the aircraft and without the piston bottoming in the cylinder. However, with conventional shock struts, damping rate during taxi is often not optimum, with the result that passenger comfort may be compromised or, worse still, damage to the strut and/or the aircraft may occur should the piston bottom out.
Yet another consideration affecting strut design is the tail-down attitude of the aircraft during takeoff. One of the objectives of aircraft design is to locate the landing gear so that the aircraft fuselage is essentially horizontal during ground operations and has an appropriate sill height for ground servicing, and so that the aircraft can freely reach a tail-down attitude of the order of 15 degrees during takeoff. Further, strut length must be minimized to keep weight to a minimum and to facilitate the stowing of the gear during flight. While the ground operation requirements and weight considerations dictate a relatively shorter strut, in many cases aircraft takeoff performance could be enhanced by a longer strut permitting a greater tail-down angle. Accordingly, with conventional shock struts, the design must frequently be compromised by either accepting reduced takeoff performance with a too-short gear, or by accepting reduced flight performance with a too-long gear.
Shock struts having variable damping rate have been proposed as solutions to some of the problems noted above. For example, U.S. Pat. No. 4,061,295 issued to Somm discloses a shock absorber for an aircraft landing gear in which a solenoid-operated valve within the piston is activated to reduce the size of the damping orifice after a predetermined time has elapsed following initial compression of the piston at first touchdown on the runway. The reduced orifice size causes substantially increased damping. The increase of the damping rate is timed to coincide with the completion of a first reactive displacement cycle of spring compression and reextension, so that succeeding displacement cycles are sharply attenuated by the increased damping rate. The shock absorber thus has two damping rates, and the switch from one to the other is strictly a function of time. Furthermore, the design of the shock absorber does not address the damping and spring characteristics that are required during taxi, and does not address strut length considerations for takeoff.
U.S. Pat. No. 4,973,854 issued to Hummel discloses a hydraulic shock absorber having an internal electromagnetically operated damping valve through which all of the fluid passes during compression of the piston, the fluid passing between a working chamber defined between the piston and the cylinder and a compensating chamber disposed within the piston. The electromagnet is controlled by an electronic control in response to measurements such as vehicle traveling speed, speed of compression, loading conditions, etc. The shock absorber can also be adjusted in length by pumping additional fluid into it or discharging fluid from it. The damping valve is actuated by fluid pressure, the electromagnet influencing only the closing force of the damping valve counteracting the fluid pressure. Thus, in the event of a power failure, the damping rate of the shock absorber would vary according to the fluid pressure, with high compressive forces being damped less and low compressive forces being damped more, which may be undesirable.